Katherine Milhous created some of the most distinctive posters produced by the WPA. She was a supervisor of the FAP in Philadelphia (her birthplace and lifelong home) from 1935 to 1940. Milhous often incorporated the folk traditions of the Pennsylvania Dutch communities in her poster designs. Her deep affection for the locality's history and people is apparent in her depictions of the Amish and Mennonites.

Milhous was educated at the Philadelphia Museum School of Industrial Art and the Pennsylvania Academy of the Fine Arts before joining the FAP. An exhibition of her posters in a FAP gallery attracted the attention of a children's book editor at Charles Scribner's Sons, where she as a staff designer 1944 - 1946, and launched her successful career as an award-winning children's book illustrator.

In 1938 her first book as writer and illustrator, Once Upon a Time, was published. It was followed by other titles including Snow Over Bethlehem (1945), With Bells On (1955), and Through These Arches: The Story of Independence Hall (1964). She received the American Library Association's Caldecott Medal, honoring the most distinguished picture book for children for The Egg Tree (1950).

Milhous was a member of the American Institute of Graphic Arts. Her work was exhibited at the 1939 New York World's Fair and at the Pennsylvania Academy of the Fine Arts.